Humanity Confronts Its Handiwork: An Altered Planet

By WILLIAM K. STEVENS

Published: May 5, 1992

HUMANS have always exploited nature in the belief that the all-encompassing biosphere -- the seamless, wondrously resilient fabric of life, land, water and air -- was so vast and enduring that people could never do it basic harm.

Events of the last decade have shattered that comforting perception. The moment of awakening may have come in the mid-1980's when governments finally accepted and acted on the evidence that waste industrial chemicals were weakening the stratospheric ozone shield that protects living things from biologically harmful ultraviolet rays.

But that is almost the least of it. People have now transformed the biosphere on so many fronts, scientists say, that Homo sapiens rivals grand forces like the movement of continents, volcanic eruptions, asteroid impacts and ice ages as an agent of global change.

The transformation has sharply escalated in both scale and pace since World War II. And it is raising serious questions not only about the ability of nature to sustain the global economy, but also about the health and future of the biosphere itself. That is why delegates from around the world are preparing to gather at an "Earth Summit" early next month in Rio de Janeiro.

There are many reasons for concern:

*People have transformed or manipulated ecosystems constituting about half the planet's ice-free land surface and have made a significant impact on most of the rest.

*They have appropriated to their own use about 40 percent of the photosynthetic energy produced by plants.

*They have steadily reduced the number of other species in the world through pollution, hunting and destruction of natural habitat. Now, as the inroads become deeper and more widespread, many biologists fear that human activity could bring about a mass extinction of epic scale, wiping out 25 percent of the world's remaining species in the next 50 years.

*By burning coal, oil, natural gas and trees they cut down, humans have altered the global flow of energy within the biosphere. Atmospheric concentrations of heat-trapping carbon dioxide have increased by 25 percent since pre-industrial times. That is well above levels recorded at any other time in the last 160,000 years, which is as far back as scientists have been able to track the trend. No end to the buildup is in sight. If it remains uncontrolled, scientists warn, a disruptive and possibly catastrophic warming of the earth could take place.

*Global population, which stood at 2.5 billion only 40 years ago, is expected to reach 6 billion by the year 2000 and swell to perhaps 10 billion 60 years from now.

Delegates to Rio hope to sign legally binding treaties to cope with the threats of climate change and species extinction. The final round of talks on climate is to conclude at the United Nations in New York this week. The last round of biodiversity talks starts next week in Nairobi, Kenya.

The Rio delegates also hope to adopt a statement of principles and an agenda for action to prevent broad damage to the biosphere while accommodating the economic needs of the surging population.

The question of limits to economic and population growth has come to the fore once again in connection with the summit. But this time it has been joined by the newer and more pressing question of the degree to which the biosphere is in jeopardy. Scientists are still struggling toward an answer in each case.

Until now, humans have always been able to push back the physical limits imposed on their expansion by the rest of the biosphere. When limited food supplies have threatened to check the proliferation of people, for instance, people have employed technology and social organization to grow crops and then clear forests, plow grasslands and, finally, harness science to agriculture. Humanity has not repealed the laws of ecology, but it has bent them; it continually expands to fill its ecological niche, then stretches the niche -- at some cost to other elements of the biosphere. Capacity for Growth

Is the capacity of the earth to accommodate human growth infinite?

No, argue Donella H. Meadows, Dennis L. Meadows and Jorgen Randers, three authors of the "limits to growth" thesis propounded 20 years ago under the sponsorship of the Club of Rome, an informal group of academics, civil servants and business leaders. The thesis, based on computerized simulations of the workings of the global ecosystem, held that limits to growth on the planet would be breached within a century and that a sudden and uncontrollable decline of population and industrial capacity would ensue. Damage to the biosphere's natural resources were one element in the predicted collapse.

In a new book called "Beyond the Limits" (Chelsea Green Publishing Co.), based partly on updated computer simulations, the three authors argue that if human activity continues as at present, it will "overshoot" the carrying capacity of the biosphere and precipitate a collapse within the next few decades.

The result, they write, would be "a permanently impoverished environment and a material standard of living much lower than could have been possible had the environment never been overstressed."